The expression "digital coding with embedded subcode", or more simply "embedded coding", indicates that within a bit flow forming the coded signal, there is a slower flow which can be still decoded giving an approximate replica of the original signal. Said codes allow coping not only with accidental losses of part of the transmitted bit flow, but also with the necessity of temporarily limiting the amount of information transmitted. The latter situation can occur in case of overload in packet-switched networks, e.g. those based on the so-called "Asynchronous Transfer Mode" better known as ATM, where a rate limitation can be achieved by dropping a number of packets or of bits in each packet. By using an embedded code, at the destination node the original signal is recovered, although at the expenses of a certain degradation by comparison with reception of the whole bit or packet flow. This solution is simpler than using a set of coders/decoders with different structure, operating at suitable rates and driven by network signalling for the choice of the transmission rate.
Among the systems used for speech signal coding, PCM (and more particularly uniform PCM with sample sign and magnitude coding) is per se an embedded code, since the use of a greater or smaller number of bits in a codeword determines a more or less precise reconstruction of the sample value. Other systems, such as e.g. DPCM (differential PCM) and ADPCM (adaptive differential PCM), where the past information is exploited to decode the current information, or systems based on vector quantization, such as analysis-by-synthesis coding systems, are not in their basic form embedded codings, and actually the loss of a certain number of coding bits causes a dramatic degradation in the reconstructed signal quality.
Coding-decoding devices based on DPCM or ADPCM techniques modified so as to implement an embedded coding are described in the literature. E.g., the paper entitled "Embedded DPCM for variable bit rate transmission" presented by D. J. Goodman at the Conference ICC-80, paper 42-2, describes a DPCM coder-decoder in which the signal to be coded is quantized with such a number of levels as to produce the nominal transmission rate envisaged on the line, while the inverse quantizers operate with the number of levels corresponding to the minimum transmission rate envisaged. The predictors in the coder and decoder operate consequently on identical signals, quantized with the same quantization step. The resulting quality degradation has proved less than that occurring in case of loss of the same number of bits in conventional DPCM coding transmission. The paper also suggests the use of the same concept for speech packet transmission, since bit dropping causes a much lower degradation than packet loss, which is the way in which usually a transmission rate is reduced under heavy traffic conditions.
In the paper entitled "Missing packet recovery of low-bit-rate coded speech using a novel packet-based embedded coder", presented by M. M. Lara-Barron and G. B. Lockhart at the Fifth European Signal Processing Conference (EUSIPCO-90), Barcelona, Sep. 18-21, 1990, a speech signal embedded coding system is disclosed which is just studied for packet transmission in order to limit degradation in case of loss or dropping of entire packets instead of individual bits. The general coder structure basically reproduces that of the embedded DPCM coder described in the above-mentioned paper by D. J. Goodman. The system is based on a classification of packets as "essential" and "supplementary" and the network, in case of overload, preferentially drops supplementary packets. For such a classification, a current packet is compared with its prediction to determine the degradation which would result from reconstruction at the receiver, the degradation being expressed by a "reconstruction index". The reconstruction index is then compared to a threshold. If the comparison indicates high degradation, i.e. a packet difficult to reconstruct, the packet is classified as "essential, otherwise it is classified as "supplementary". The two packet types are coded and transmitted normally through the network. The derision "essential packet" or "supplementary packet" determines the position of suitable switches in the transmitter and receiver in such a manner that, at the transmitter, after transmission of a supplementary packet the predicted packet is coded instead of the original one, and the coded packet is also supplied to a local decoder and a local predictor in order to predict the subsequent packet. At the receiver, essential packets are decoded normally and supplied to the output. A local encoder is also provided for updating the decoder parameters in case of a missing packet, by using a packet predicted in a local predictor. A supplementary packet is decoded and emitted normally, but it is supplied also to the local predictor and encoder to keep the encoder parameters in alignment with the encoder parameters at the transmitter.
DPCM/ADPCM coding systems offer good performance for rates basically comprised in the interval 32 to 64 kbit/s, while at lower rates their performance strongly decreases as the rate decreases. At lower rates different coding techniques are used, more particularly analysis-by-synthesis techniques. Yet, also these techniques do not result in embedded codes, nor does the literature describe how an embedded code can be obtained. The paper by M. M. Lara-Barron and G. B. Lockhart states that the suggested method can also be applied to any low-bit rate encoder that utilises past information to decode current-frame samples, and hence theoretically such a method could be used also in case of analysis-by-synthesis coding techniques. However, even neglecting the fact that indications of performance are given only for 32 kbit/s ADPCM coding, the structure of transmitter and receiver is the typical structure of DPCM/ADPCM systems, comprising, in addition to the actual coding circuits at the transmitter and decoding circuits at the reciever, a decoder and a predictor at the transmitter and a predictor at the receiver. The devices are not provided for in the transmitters/receivers of a system exploiting analysis-by-synthesis techniques, and their addition, besides that of the circuits for determining the reconstruction-index, would greatly complicate the structure of said transmitters/receivers. Furthermore, since the coding/decoding circuits comprise a certain number of digital filters, the problem arises of correctly updating their memories.